Abstract
A time series of experiments at high temperature have been performed to investigate the influence of particle settling on magma mixing. A natural rhyolite glass was held above a natural basalt glass in a platinum crucible. After melting of the glasses at superliquidus temperatures, a platinum sphere was placed on the upper surface of the rhyolitic melt and sank into the experimental column (rhyolitic melt above basaltic melt). Upon falling through the rhyolitic–basaltic melt interface, the Pt sphere entrained a filament of rhyolitic melt in its further fall. The quenched products of the experiments were imaged using X-ray microCT methods. The images of our time series of experiments document the formation of a rhyolite filament as it is entrained into the underlying basalt by the falling platinum sphere. When the Pt particle reached the bottom of the crucible, the entrained rhyolitic filament started to ascend buoyantly up to the initial rhyolitic–basaltic interface. This generated a significant thickness increase of a comingled “melange” layer at the interface due to “liquid rope coiling” and piling up of the filament. As a consequence, the basalt/rhyolite interface was greatly enlarged and diffusive hybridisation greatly accelerated. Further, bubbles, originating at the interface, are observed to have risen into the overlying rhyolite dragging basalt filaments with them. Upon crossing the basalt/rhyolite interface, the bubbles have non-spherical shapes as they adapt to the differing surface tensions of basaltic and rhyolitic melts. Major element profiles, measured across the rhyolite filaments, exhibit asymmetrical shapes from the rhyolite into the basalt. Na and Ti reveal uphill diffusion from the rhyolite towards the interface in the filament cross sections. These results reveal the potential qualitative complexity of the mingling process between rhyolitic and basaltic magmas in the presence of sinking crystals. They imply that crystal-rich magma mingling may be expected to be accelerated with respect to crystal-poor systems. We urge the further fluid dynamic analysis of these phenomena, obtainable for the first time using detailed tomographic imaging.Electronic supplementary materialThe online version of this article (doi:10.1007/s00410-016-1305-1) contains supplementary material, which is available to authorized users.
Highlights
Magma mixing is the combined process of physical mingling and diffusive chemical mixing between end-member magmas
The time series of three experiments allows the observation of the mingling process with time (Fig. 1)
The experiments demonstrated particle-induced mixing to function to viscosity contrasts of up to 4000
Summary
Magma mixing is the combined process of physical mingling and diffusive chemical mixing between end-member magmas. The mingling component of mixing is a physical dispersion of the magmas, whereas the diffusion is in response to the potential gradients of the diffusing species and acts to homogenise the mixture by decreasing chemical gradients (Perugini and Poli 2012). The combination of mingling (which enlarges melt interfaces) and chemical mixing (diffusion between melts) is summarised under the term chaotic mixing Contrib Mineral Petrol (2016) 171:96 et al 2003; De Campos et al 2011) Such mingling results in a chaotic, three dimensional dispersion of two coexisting fluids, forming active regions of the development of filaments and coherent regions reminiscent of enclaves (Perugini et al 2002). The degree of chaos depends on the mingling/mixing mechanism and rheological properties and forms variable structures of varying complexity
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